In The Hunger Games: Catching Fire, Romulus Thread threatened to kill the District 12 victors. In the end, he instead imposed martial law. However, is there any indication as to what would have happened to him if he'd actually killed the victors? Would he have been rewarded, or executed?
While we can't know the answer to your question for certain, we have enough information to be able to work out quite a few things and come to a probable conclusion.
Firstly, Thread thinks he would be punished for killing them.
“I don't care if she blew up the blasted Justice Building! Look at her cheek! Think that will be camera ready in a week?” Haymitch snarls.
The man's voice is still cold, but I can detect a slight edge of doubt. “That's not my problem.”
“No? Well, it's about to be, my friend. The first call I make when I get home is to the Capitol,” says Haymitch. “Find out who authorized you to mess up my victor's pretty little face!”
-- Catching Fire, Chapter 8
Haymitch acts confident, threatening Thread with repercussions from the Capitol, and after his fellow Peacekeepers offer him a way out, Thread backs down. Clearly Thread thinks he isn't authorised to kill or even beat the District 12 victors.
Given this, let's ask ourselves: what were Thread's orders for District 12? Obviously he was ordered to crack down hard on the population, perhaps to impose martial law on the district. But given his uncertainty in the face of Haymitch's words, he apparently wasn't given very clear orders on how to deal with the victors. His mission was to subdue the common people of District 12; dealing with Hunger Games victors is above his pay grade. This is why he's afraid to do anything to them, and backs down when they stand up to him; Snow hasn't authorised him to deal with them.
Now let's try to look at this from the point of view of Snow and the Capitol.
Why didn't he authorise Thread to deal with the victors in District 12? Because Thread was the wrong tool for the task. A brutish Peacekeeper is all very well for subduing the general population, but to deal with people like Katniss and Haymitch, more careful hands are needed. Having Thread kill or attack the District 12 victors was never part of the plan.
What would he have done if Thread had killed them? Ah, now we get to the real nub of it. My opinion is that it depends on the outcome, for which there are some very different possibilities. (This uncertainty in outcome is precisely why Snow didn't authorise Thread to carry our such an act.)
If Katniss's death had snuffed out the flame of revolution, and the people had subsided back into their abject state when deprived of their Mockingjay figurehead, Snow would have been pleased with Thread's action. Even if he had to reprimand the man publicly in order to keep up the pretence that victors are honoured citizens and killing them is unthinkable, I'm sure Thread would secretly have been rewarded.
If, on the other hand, Katniss was held up as a martyr and her death used to fan the flames of revolution to still greater heights, then Thread would probably have gone the same way as Seneca Crane. We already know what Snow can do to those who displease him and help to cause revolutions, no matter how inadvertently, after he gives them positions of power.
(EDIT: well, not quite the same way as Seneca Crane. The latter was quietly 'disappeared', and only those with sufficient intelligence and insight into the Capitol's methods could guess what had happened to him. Thread, on the other hand, would more likely have been publicly executed. Snow would have wanted everyone to know Thread was acting alone and didn't have presidential orders to kill Katniss. Once she was dead, he could act as sad over her as everyone else, in order to try to appease the population, and punish her killer as publicly as possible. Thanks to @DVK for pointing this out.)
The question remains of which of these would have happened. If Katniss was truly the only figurehead, and the districts were isolated and disorganised in their uprising, the first option would probably have been more likely. But given that Coin and District 13 must have been pulling the strings of revolution even at this early stage, they would probably have found advantage in her death just as they did in her life. Recall how when Katniss was presumed dead in Mockingjay, Coin immediately tried to turn the situation to her advantage by holding her up as a martyr.
Note also that being killed by a Peacekeeper is a very good way to become a martyr. Snow wants Katniss dead, but he wants her dead in a way that benefits himself and not the revolutionary cause. When he does try to arrange her death, he does so in a way that makes her seem allied with the Capitol - going back into the Hunger Games, becoming once more "a piece in their Games" - rather than allied with the common people against the Capitol and its authority. The revolutionary elements would love her for standing up to a Peacekeeper, and love her all the more if she was killed doing so, but Snow's plan is for them to hate her for becoming part of the Capitol's schemes, and hate her even when she dies as a piece in their Games.
Summing up: the most likely possibility is that if Thread had killed the District 12 victors, they would have been turned into martyrs and used for the revolutionary cause, and he would have been publicly executed in a Capitol attempt to reconcile with the rebels.